1. Field of the invention
The invention relates to a process for treating ferrous metal parts in order to improve their corrosion resistance in which the parts, which are also subjected to heat and chemical treatment including nitriding in association or not with carburizing and/or sulfurizing, are immersed in a bath of molten salts.
2. Description of the prior art
British Pat. No. 2,056,505 describes a bath of salts comprising alkali hydroxides and from 2 to 20% by weight of alkali nitrates. Immersed in it are ferrous metal parts which have been subjected to nitriding in a bath of molten salts including cyanides. The effect of the bath containing hydroxides and nitrates is firstly to destroy the cyanides and then, by further action, to improve the corrosion resistance.
U.S. Pat. No. 4,448,611 describes a bath of molten salts made up by adding to a known oxidizing bath from 0.5 to 15% by weight of oxygen-containing salts of alkali metals for which the normal oxidation-reduction potential relative to the hydrogen reference electrode is -1.0 V or below. The bath, intended in particular for treating ferrous metal parts containing sulfur in their surface layers, in order to improve their corrosion resistance, is implemented by blowing in an oxygen-containing gas, limiting the concentration of insolubles to below 3% by weight of the bath.
The state of the art process for improving corrosion resistance is essentially based on the formation on the surface of the parts of an adherent barrier layer of a stable oxidized compound, that is to say one featuring a high formation energy.
The use of oxidizing salt baths features inherent hazards of attack on the surroundings of the bath, pollution and explosion, these hazards being proportional to the oxidizing power of the bath, that is to say in the final analysis to the degree of corrosion protection achieved.
It therefore appears desirable to provide processes for treating ferrous metal parts in order to improve their corrosion resistance which are at least as effective from this point of view as the conventional oxidizing baths whilst not having the disadvantages associated with their oxidizing power, or having such disadvantages to a reduced degree in the event that the use of an oxidizing bath proves necessary to achieve a result other than corrosion resistance.